1. Field of the Invention
The invention relates generally to data communication systems and more particularly to a data communication system for broadband coaxial cable which supports a plurality of communication channels.
2. Description of the Prior Art
Data communication is the transfer of digital information from one point to another, usually between a user terminal and a computer. In more sophisticated systems communication may take place between two computers, a computer and a remote printer or between combinations of computers, memory devices, terminals, or microprocessors used to control various devices, e.g. burglar alarms, to traffic controls, industrial operations, etc. Data is transmitted through a communication channel by a transceiver, sometimes referred to as as MODEM. Generally, the channel composes a pair of wires but can also assume other forms, e.g. radio frequency channels, microwave transmission channels, or optical fibers. In many applications a channel has a device at each end such as a computer connected to a terminal by a pair of wires. Data communication is also possible where there are more than two devices communicating on a channel.
When more than two using devices are connected to a single communication channel there are two methods of data communication. The first method is to assign time slots to the using devices connected to the channel. A using device may then transmit data during its time slot and must wait until the next cycle to transmit again. This method is known as "time division multiple access." The second method of communication allows any using device to transmit data when the communication channel is not being used by another device. This method of communication is referred to as "carrier sense multiple access (CSMA)."
A problem encountered with CSMA communication systems is that data collision occurs when two devices begin to transmit data at the same time. A signal sent by one transceiver will reach another transceiver a short time later, thus during the interim a second transceiver may begin transmitting a signal. The two signals will collide causing the data to be garbled.
Several methods have been developed for collision detection. One method, described in U.S. Pat. No. 4,063,220, granted to Metcalfe, et al., is to compare concurrently the cable contents bit by bit against the locally transmitted contents. If a difference is detected the transmission is aborted. Another method uses a looped cable where a signal is transmitted on an inbound cable and received on an outbound cable. A transceiver will receive its own transmission after the propagation delay of the cable. Once the entire transmission is received it is then compared to the transmitted signal to determine if the data has collided with another transmission. This method is inefficient in that a collision is not detected until the entire transmission is received.
When a collision has been detected by one of the above means the transceiver stops transmitting and backs off before retransmitting the same data. In order that both transceivers do not restart transmission at the same time the backoff time is determined either randomly or by a predetermined unique time interval.
In the prior art, data communication systems for coaxial cable have been limited to communication on a single channel. A channel is characterized by its position within the spectrum available (frequency) and the space it requires (bandwidth). Baseband coaxial cable has long been used for data communications. However, due to its physical characteristics only one communication channel is practical. Broadband (or CATV) coaxial cable has been used less frequently for data communication although it has long been used for cable television. The use of broadband coaxial cable for data communication is discussed in the article "Broadband Technology Magnifies Local Networking Capability" appearing in Data Communications, Vol. 9, No. 2, February, 1980. On broadband cable a broader frequency spectrum is available for transmission thus it is possible to divide the spectrum into frequency bandwidths which can be used for separate channels.
Information is communicated by a MODEM. MODEMs for use on broadband cable must be designed to transmit signals only in the frequencies of the channel to which it is assigned. Otherwise, it will cause interference on the other channels. Data is encoded by modulation which is accomplished by a variety of techniques. A problem encountered with modulation is that signals outside the bandwidth of the designated channel are generated. Therefore, it has been necessary to use filters. Filters add greatly to the expense of a transceiver and limit the transceivers operation to a designated channel. Thus, in the priot art, MODEMs for data communication over coaxial cable have been limited to operation on a single communication channel.